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Here's what I want and here's what I got (mild 400 build)

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Here's what I want and here's what I got (mild 400 build)

zenwren zenwren
New User | Posts: 4 | Joined: 06/14
Posted: 06/17/14
12:14 AM

Hi everyone, new guy here, I didn't see an intro thread area so I'll just dive right in...
I've got a 1970 Firebird with the factory 350 and I'm thinking of upgrading to a 400. I've got a 568557 engine that needs a rebuild and two pairs of heads 6x8's and 6x4's (also the 11's from my 350) I'd like to build something around 350-400hp. Why 350-400hp? I have no idea, that just seems like a reasonable amount to ask for. I mostly use the car for cruising, some highway, some around town, and of course a little screwing around from stop light to stop light.
 I'm thinking the way to go is with the 6x4's, I'd like to be able to run 87 but could spring for the 91 octane if it's going to be necessary. My initial questions are,

What kind of static compression ratio should I be looking at to get me to my HP goal?

Should I consider decking the block, milling the heads, porting, or finding a different set of heads all together?

Is my 568557 block a glorified boat anchor and I should be looking for a stronger earlier block?

Are there any good resources for tried and tested builds I can copy?
Would I be better off with a 455? (I have access to a 71-72 builder for around $550)

I should also mention that my car has a TH350 and 2.73's I would be willing to upgrade the trans and rear ratio depending on what I end up building, possibly with an overdrive. I'd just like to have a fun streetable hot rod.

Thanks in advance,

My71 My71
Guru | Posts: 1261 | Joined: 02/10
Posted: 06/17/14
05:09 AM

welcome to the forum.
You can never go wrong using an earlier stronger block.
The 6x heads have a bigger intake valve than your 11's. the 8's are about 101 CC and the 4's are about 93 CC.
Your HP goal is hard to call by considering CR alone. Best bet is to purchase a dyno-sim program and play with different factors to see what your numbers look like. For the street, I'd pay attention to the torque numbers more than the HP numbers.
A quick hit for the street would be to upgrade your rear end from that 2.73 to a 3.23 or higher.  

bigD bigD
User | Posts: 155 | Joined: 05/14
Posted: 06/17/14
09:15 AM

I vote for the 455 with the 6x-4 heads, a Melling SPC-8 cam with Rhoads RL9518 lifters, and a GOOD Q-jet. It needs at least SP L2359NF30 forged pistons with 5140 or better forged rods. Here's a similar build you can check out. It's a crate motor, that cost $4500 + shipping, with Eagle crank and rods.

Or here's a 4.25 stroker short block, to which you can add your own heads, cam, etc.

With one of these, you won't need any more rear end gear. But you will have to get a heavy duty TH350 or TH400. The long stroke Pontiac will eat the stock sprag in the TH400 and the stock sprag race in the TH350. It will cost between $1200-$1600 for a tranny that will survive a 455+ Pontiac. That is unless you have a GOOD tranny guy in your area who knows how to do this build and will do it cheaper than the online price.

Of course, you can do it with a 400 or even a 350, but it will take more rear end gear and a stall converter, to get the same launch. Don't use any 400 block that ends with the numbers "557" !

This is just a few general ideas. If you need details about any of it just ask. And, as always, you'll find that opinions differ, here, just as on other forums.  Smile  

zenwren zenwren
New User | Posts: 4 | Joined: 06/14
Posted: 06/17/14
09:32 AM

While I admit the 455 is beginning to tempt me my main obvious concern is the fuel economy. What sort of difference in cost at the pump could I expect from a built up 400 vs a more mild 455? Is the "weaker" 3.25" main in a 455 block really anything to be worried about if I'm not racing the thing? Would I be able to run 87 octane on 6x4's on a .30 over 455? Maybe with dished pistons?  

bigD bigD
User | Posts: 155 | Joined: 05/14
Posted: 06/17/14
10:14 AM

OK, if your main concern is gas mileage, then that changes everything ! You need to build the 350 with SP 357P30 pistons, 5140 forged rods, Summit 2800 cam kit, and the #11 heads. Don't shave the heads or deck the block--no more than a clean up cut on each. You need to leave the CR as low as possible, so you can still run the cheaper 87 octane gas. Run a Q-jet carb.

You also need to keep the timing low, like maybe no more than about 30-32 degrees total, to reduce the chance of detonation with the 87 octane. And don't run the light advance weight springs. You don't want the timing to advance too quickly, again, to discourage detonation. Again, this is just the basics.

Keep the 2.73 gears. Buy a good 200R4 tranny. This will give you a slightly lower 1st gear ratio, but overdrive in high gear. It's the best gas mileage you can get.  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 496 | Joined: 11/13
Posted: 06/17/14
10:35 AM


ok my71 is a sharp guy and is being kind. i will not be unkind but i will be briutaslly homest. what you want is titally unrealistic and ain't never ever gonna happen.

the less compression you have the harder it is ti make hp and the FASTER you must turn your engine to reach your 400 hp goal. the faster you turn your engine, the more gas it eats, therefore its a catch 22 situation.

here are a few realistic options.

1. build an engine for fuel economy and forget about hp.

2. throw mileage out the window and plan to buy 91 octane and get around 9 mpg in town and 12 on the highway.

3. buy an LS1 engine with the computers and get 18 mpg and around 375 hp.  

zenwren zenwren
New User | Posts: 4 | Joined: 06/14
Posted: 06/17/14
11:04 AM

Whoa whoa whoa fellas I never said a was trying to build a gas sipper here! My 350 currently gets 17mpg which is awesome. I realize I'm going to have to sacrifice economy for performance but I'm not going to just throw it out the window as a non-issue. I've been told I can expect around 8mpg with a 455, is there no middle ground here? I can live with anything over 12mpg but I really don't know what you can get away with at 87 vs 91.
The bottom line is a pretty common one, I want a fun, reliable car that can pass most things, including the occasional gas station.
Franky I doubt my current set up would be very happy turning ~1500rpm on the highway with a 200r4.  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 496 | Joined: 11/13
Posted: 06/17/14
11:31 AM

well my 71 mentioned changing gearing. this will help a lot if you are running like 280 or 3.23.

if you increase your hp you need to change your gearing anyway.

we can not say, ok if you want 15 mpg and you have 3.23 gears with an od trans you will get 15 mpg at 65 mph with a 324.7 hp engine because most of us are not that smart, lol.

most of us either cruise or want a good amount of hp and don't worry about mileage.

what i'm saying is. if you want 400 hp in any engine, we can tell you how to get there. if you want the most hp you can get at 15 mpg at 65 mph we have absolutely no clue other than the ls1 engine i suggested.

computer controlled tpi fi and ignition timing will get you the best mileage vs hp. you can buy this for your engine but it's probably $4000.00.

If you have an od trans, my GUESS would be, 325 hp in a 400 ci engine when driven CONSERVATIVELY at at least 10 mph in town and at least 13 on the highway.

we can get you to 325 easy.

here's some std questions.

1. how much money do you have for the engine and carb? this does not include gearing or exhaust or ignition.

do you want a mild, moderate or nasty idle?

fwy flier, lite to lite racer or inbetween?

notice that "how many mpg do you want?" is not one of them, lol.

MILEAGE/HP IMPROVERS - To get the most mileage AND performance.

square and 0 deck block - adds 1/4 - 1/2 mpg?

annular boost venturi carb - adds 1/4 - 1/2 mpg? over non annular type

thermal coat combustion chamber - adds 1/16 - 1/8 mpg?

msd 6al-2 ignition box - adds 1/16 - 1/4 mpg?

hei ignition - adds 1/16 - 1/8 mpg?

properly set up distributor timing curve with a dist vacuum booster - ads 1/8? mpg over improperly set up one.

192 thermostat, this is stock and a warm engine gets better mileage than a cool one like a 180 t stat - adds 1/16 mpg over 180 t stat

water and methanol injection if properly set up - adds 1/4 - 2 mpg? around 1/2 in your case

PS - The name of the site is high perf pontiac. maybe it should be high perf gas guzzlin pontiac, lol.  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 496 | Joined: 11/13
Posted: 06/17/14
12:59 PM

Ok look at this


400 ci .030" over

Square block

Make piston to deck height .003" - .005".

HEADS – buy different heads.

If you don’t want to buy other heads then use your 11 heads because they have small combustion chambers. Make the chambers 78 cc’s. They should be close to 78 cc’s stock. If they are smaller, you can have them enlarged for around $200.00.

Run your stock valves or install the big Pontiac valves or better yet, buy 1968 Pontiac 350 1.96 intake valves or better yet, Chevy 2.02 intake valves and 1.71 Ford ex valves from Manley.

Have heads “pocket ported”.

3 angle valve job.

back cut intake valves.

Arp screw in studs.


Manley pg 31.

Install Chevy guide liners in exhaust.

Run longer push rods on exhaust.

Cleveland  "Racemaster".

1.71   stem 3.415   length 5.042   tip .250

Pontiac 350 stock.

1.66   stem 11/32 [3.437]  length 4.88?   tip .250?

CAM OPTIONS - The DEH might give the best mileage but still give "good" perf.

Comp XE262h
Comp XE268h
Compe 265DEH

EXHAUST – Headers or nice cast iron RA IV manifolds. 2 1/2" pipes are more than big enough.

ROCKERS - Scorpion "race" model full roller.


CARB - 650 - 700 cfm. Annular booster type holley style. preferred. Vacuum secondaries will provide slightly better mileage than mechanical secondaries. Mechanicals are WAY more fun but you pay every time you floor it because your mileage will drop to around 2 mpg, lol.

GEARS – 3.31 for fwy or around 3.55 for more street.

DISTRIBUTOR - Rebuilt HEI from Autozone etc. and recurve it, or new fancy MSD or Mallory with vacuum chamber.


STALL CONVERTER – If you want a fwy flier but decent off the line acceleration you can put a bigger stall conv in it. A conservative one is 2000 and will be mildly noticeable. A moderate one is 2400. I would not go over 2400.

The engine will pull pretty strong off bottom and mid and fall on its face around 5k. I will not be a rocket but certainly will not be wussy either and IF you do everything above, you "should" get at least 12 mpg fwy with an of trans. It will also likely need 91 octane. You might get buy with 89 or 87 if its E15 and you reduce the timing a little.  

zenwren zenwren
New User | Posts: 4 | Joined: 06/14
Posted: 06/17/14
06:15 PM

I must be missing something, I thought the 6x4 heads were a good choice for a 400? Are my goals too high for them? Like I said a just pulled the 350-400 out of my rear as a guess at what I could shoot for, so maybe I'm aiming a little too high? If I throw the 350hp goal out, what should I shoot for with the 6x4's? I know plenty of people out there are running them on 400's.  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 496 | Joined: 11/13
Posted: 06/17/14
06:34 PM

nope because the combustion chamber is too big so your compression will be low. plus the valves are too big to get you the mileage you are looking for with any type of hp because the air velocity will be too low.

its complicated. the package i put together meets your goals. it is very specific. not just a bunch of junk tossed together.

if you run 6x4 heads you need a 428 or 455 or custom made $800.00 pop up [dome]forged pistons in a 400 or you can pay around $120.00 to mill .100" [around 3/32"] off both of them then spend around $180.00 to machine your brand new intake so it fits.

unfortunately i can't explain all the theories/principles of engine building because it would fill a book.

you need to answer the questions or we can not help you and also be sure to read everything.  

bigD bigD
User | Posts: 155 | Joined: 05/14
Posted: 06/18/14
03:14 AM

Icon Quotezenwren:
I must be missing something, I thought the 6x4 heads were a good choice for a 400? Are my goals too high for them? Like I said a just pulled the 350-400 out of my rear as a guess at what I could shoot for, so maybe I'm aiming a little too high? If I throw the 350hp goal out, what should I shoot for with the 6x4's? I know plenty of people out there are running them on 400's.

I'll add a few words about the 6x-4 heads. As most guys know, they were used on the last Pontiac 350 engines and the Y72 400 engines the last couple of years of 400 production. They flow OK and have 2.11 intake valves. But they only made around 7.6 CR on the 350 and just over 8:1 on the 400. The more comp a motor has, the more power it can make. At only 8:1 CR, it's hard to make any serious power with a 400.

Now these heads would have been great on the last 455, back in '76. But Pontiac was only interested in fed emission requirements and lowering the overall hp average of the Pontiac brand. So, the 455 had HUGE chambers of about 124cc !   Shocked

Because of this, the last 400's had only about 8:1 CR and a 220hp rating. Now, if you wanna keep your 6x-4 heads on a 400, then there are a few things you can do to increase performance. Most of these things concern increasing comp.

(1) Most factory pistons, were designed to run about .020 down in the hole. To increase CR, you can have the block deck cut down to make the top of your pistons even with the deck at top dead center. This, of course, is called "zero decking" the block. But, to allow for a small cut on a future build, should it be necessary, I would leave the piston from .005 to .010 in the hole.

(2) Next, to increase CR you must have some milled of the heads. If you know the heads have never been cut before, then you can cut at least .060 off. Many say more is safe. But some say when you cut too much off, it can cause overheating problems. The only way you can know how big your combustion chambers are is to measure or "cc" them. Most sources agree that the combustion chambers of the 6x-4 head were somewhere in the low to mid 90's. You need to get them well down into the 80's. Also it is very important to cut exactly the same amount from the intake surface of the HEAD(not the intake manifold), as you cut from the chamber surface of the head. Also include in this cut any amount you cut from the block deck. This cut is necessary because everything that is cut off the block and the chamber surface of the head will lower the intake manifold mounting holes.

If you cut a bunch off the chamber side and the deck without cutting the same off the intake side, then the bolt holes in your intake manifold will not line up with the threaded intake bolt holes in the heads. You could cut your intake manifold to fit. But if you do that, then that manifold will not fit other engines. And, if you should decide to try another intake on your motor in the future, it will also have to be cut before it will fit, even if it is new, just out of the box.  Frown

Some brands of motors require different amounts taken off the two head surfaces. But Pontiacs are said to be what are called 90 degree motors. See if you can tell why from these pics.

(3) Thinner head gaskets will also increase CR slightly, as will head gaskets with a smaller bore size. The popular Fel-Pro gaskets are about .039 thick I believe. But they also have a one-size-fits-all bore size of 4.30". Cometic makes the best gaskets available and they offer 400 Pontiac gaskets in several sizes that will increase CR over the Fel-Pros. If your 400 is std, .020 over, or .030 over, then you can use the Cometics with a 4.160 bore size. But if you are .060 over, you'll need to buy the 4.20 bore gaskets. They have .027, .030, and .036 thick gaskets which will increase CR a little.

(4) If you'd rather not do all this stuff to increase CR, you can run one of the small steep ramp cams, like the CC XE series and the Lunati Voodoo series. They are designed to increase cylinder pressure and thus increase performance some. Or, you can use a small factory grind cam like an 067 or 068 with Rhoads lifters. This combo is said to act like a steep ramp cam, but has less lift, which should increase cylinder pressure and get slightly better gas mileage. Cliff Ruggles says the only down side to Rhoads lifters is the "ticking" sound they make below 3000 rpm.

(5) Another way to increase performance and fuel mileage at the same time is to use lite weight components--not only in the engine, but every pound of weight you can reduce from the whole car, will take slightly less fuel to propel from point A to point B. Maybe you don't have any extra junk in the trunk, but you can at the very least cruise around town with half a tank of gas instead of full. Also, unless you are racing or taking sharp curves faster than you should, you can save some gas by running narrower tires which put less footprint on the road. The big fat tires look cool, but the larger the footprint, the more friction it produces, therefore the more gas it uses. Underinflated tires will also use more fuel. Wheels come in different weights. The lighter the wheel/tire combo, the less gas it will take to turn 'em. Dragging brakes will reduce gas mileage. Sometimes older calipers will not release as fully as they should. Any amount of extra brake drag will increase fuel consumption. All these things combined can make a difference.

(6) Lighter engine parts: The 1st thing that comes to mind is the intake manifold. An alum one without the heat crossover is much lighter than a stock cast iron unit. An alum air cleaner is lighter than a chrome one. Thin alum valve covers are lighter than chrome or cast. And alum heads are much lighter than stock iron heads. Thin, cheap headers are lighter than cast manifolds and thick wall coated headers. It seems that most everything that increases gas mileage also costs $. An alum block will save lots of weight, but also cost LOTS of $. So, I reckin you'll just have to figure out if the gas savings will ever pay for the high priced parts it takes to save the gas.

A lighter rotating assembly will rev quicker, make more power, and get better gas mileage. The superlight custom pistons will reduce rotating assembly weight. Since you are not racing or using power adders, you can order pistons with extra lightening. Then you can order the lightest rods available, and even have some more lightening done to 'em. You can also have some taken off the crank counterweights, since the rods and pistons will be lighter.

A crank scraper and good windage tray will keep some of the oil off the crank, which will help some. Running a 60 lb oil pump instead of an 80 lb will help some. Also running thinner synthetic oil will help a little.

A real tight, low stall, 10" torque converter will reduce rotating weight, as compared to a stock 13" converter. A TH350 will reduce rotating weight, as compared to a TH400. And everybody knows that an overdrive tranny will reduce rpm at highway speed, thus increasing highway gas mileage.

Conclusion: After considering all this info, it is my opinion that these things are only worthwhile if your car is a daily driver that chalks up a lot of miles. Otherwise, I'd buy a gas mizer daily driver and drive the Pontiac for fun !   Smile

It is possible to build a nice 400 with 6x heads. Here is a 400 crate long block with 6x heads & 9.25 CR, that is estimated to have 385hp potential.